Photoelectric exposure time and contrast print control instrument for photographic enlargers



June 8, 1948. A. SlMMON 2,443,058

PHOTOELECTRIC EXPOSURE TIME AND CONTRAST PRINT CONTRQL INSTRUMENTS FOR PHOTOGRAPHIC ENLARGERS Filed April 25, 194'? 6 Sheets-Sheet 1 X1 I\ Q Q E A/fied fiimmon INVENTOR.

ATTORNEY June s, 1948.

A. SIMMON 2,443,053 PHQTOELECTRIC EXPOSURE TIME AND CONTRAST PRINT CONTROL INSTRUMENTS FOR PHOTOGRAPHIC ENLARGERS Filed April 25, 1947 a Sheets-Sheet 2 Fly-8 A/fred fiimmon IN VEN TOR.

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,4 TTDRNEY A. SIMMON PHOTOELECTRIC EXPOSURE TIME AND CONTRAST PRINT June 8, 1948. 2,443,058

- CONTROL INSTRUMENTS FOR PHOTOGRAPHIC ENLARGERS Filed April 25, 1947 6 Sheets-Sheet 3 LIGHT A lfred fiimmon BY Mash/14m,

. A. SIMMON 2,443,053 PHOTOELECTRIC' EXPOSURE TIME AND CONTRAST PRINT CONTROL INSTRUMENTS FOR PHOTOGRAPHIC ENLARGERS Filed April 25, 1947 6 Sheets-Sheet 4 June 8, 1948.

U m 5 MM w 5 A w W Y B m m ATTORNEY June 8 194 5? A. SIMMO PHOTOELECTRIC, EXPOSURE TIME AND CONTRAST PRINT CONTROL INSTRUMENTS FOR PHOTOGRAPHIC ENLAHGERS Filed April 25, 1947 6 Sheets-Sheet 5 I a 5 1 60 l 1 I I I I I I I /00 30& l

I i 502 200 205 I I 203 y I /87 w 182 [83 l i 90 l 257/1I 20/ 9/ L202 6;

.A/fred Sim/210m IN VEN TOR.

4 TTOR/VE Y 6 Sheets-Sheet 6 A. SIMMON PHOTOELECTRIC EXPOSURE TIME AND CONTRAST PRINT CONTROL INSTRUMENTS FOR PHOTOGRAPHIC ENLARGERS June 8, 1948.

Filed April 25, 1947 Alf 8a 5/07/110/1 IN VEN TOR. BY Mm; Wm

A TTORNE Y Patented June 8, 1948 PHOTOELECTRIC EXPOSURE TIME AND CONTRAST PRINT CONTROL INSTRU- MENT FOR PHOTOGRAPHIC ENLARGERS Alfred Simmon, Jackson Heights, N. Y., assignor to Shannon Brothers, Inc., Long Island City,

N. Y., a corporation of New York Application April 25, 1947, Serial No. 743,949

20 Claims.

The obiect oi this invention is an instrument which indicates the correct exposure time for and the contrast oi! the enlarged image of a photographic-negative as projected by a photographic enlarger upon an easel. This is accomplished by a photoelectric cell placed on this easel or, more specifically, on the points of brightest and darkest illumination thereon. The current passing the photocell in response to the light impinging upon it causes a condenser to change its voltage more or less rapidly, and the time consumed for a voltage change of a predetermined magnitude is measured. The results, respectively, obtained for the brightest and darkest spot on the easel are converted by mechanical means to indications for the exposure time and contrast. For best photographic re suits, the contrast indication is as necessary as the indication of the correct exposure time since otherwise the contrast range of the photographic negative and of the sensitized paper on which the enlarged image is printed cannot be matched and the indication of contrast is, therefore, instrumental in enabling the operator to select a paper with the proper degree of contrast for a particular print.

A preferred embodiment of my invention is shown in the accompanying drawings in which Fig. 1 shows the general appearance of the device in conjunction with an enlarger of typical design;

Fig. 2 is a plan view of the photocell unit with its cover removed;

Fig. 3 is a sectional view along the plane of line 3-3 in Fig. 2;

Fig. 4 is an electric circuit diagram;

Fig. 5 is a front elevational view of the main unit with the cover removed;

Fig. 6 is a fragmentary sectional view along the plane of line 66 in Fig. 5 showing details of the resetting device;

Fig. '7 is a sectional view along the plane of line 1-1 in Fig. 1;

Figs. 8, 9 and 10 are developments of cams associated with the two time-measuring devices for the brightest and darkest spot, respectively. By means of these cams the measured charging times of the condenser are converted into exposure time values and logarithmic light intensity values, respectively.

2 Like characters of reference denote similar parts throughout the several views and the following specification.

BASIC ELECTRICAL DESIGN circuit associated with this cell. due to the very low light intensity to be measured, represents an additional problem.

It is known that it is expedient for this purpose to use a condenser in series with the photoelectric cell and to measure the time which it takes to charge or discharge this condenser. While it is theoretically unimportant whether the condenser is charged or discharged during this process, more convenient circuit relations can be obtained by having the condenser charged and this has been shown in the following example. The current that passes the photoelectric cell is substantially proportional to the intensity of the incident light. The charging or discharge time of the condenser, however, is inversely proportional to thecurrent with which it is charged or discharged, and consequently for a circuit of this type charging or discharge times of the condenser for zero light intensity become infinite, and for low light intensities these times become very long. This is objectionable for a number of reasons, so for example, with very long charging or discharging times, incidental small leakages may falsify the result completely. In order to overcome this condition, I provide two parallel but otherwise independent charging circuits for the condenser. The current passing the first circuit is controlled by the photoelectric cell and is at least substantially proportional to the light intensity to be measured. The current passing the second circuit is constant and entirely independent of the light intensity, but may, of course, for convenience, be adjusted to a suitable value where it will be left during the operation of the device. The result of this arrange- C=condenser capacity T=condenser charging time E=condenser voltage ii=condenser charging current through multiplier tube in o. (micro-amps.) =multiplier sensitivity in Jim/foot candles =light intensity in foot candles (on easel surface) ia=condenser charging current through auxiliary circuit (in M.)

and if we assume that the condenser is charged from a zero voltage, the condenser voltage E can be expressed as follows:

For 11:0, 1. e., without any light reaching the 3 photo-electric cell, we shall call the condenser charging time To. This time becomes This can be introduced into the formula for T:

As will be seen later, based on this relationship, cams of very convenient configuration can be designed by means of which exposure times as well as logarithmic light intensity values can be obtained. The convenience with which this can be done is based on the peculiar characteristics of the double condenser charging circuit as expressed by the above formula and this is one of the principal advantages of this arrangement.

BASIC MICHANICAL DESIGN Reference to Fig. 1 shows that the unit consists, in addition to the enlarger which is merely shown in the interest of completeness, of two firgcipal units, the photo-cell unit and the main ENLARGER The enlarger may e of any convenient form or design and merely as a matter of example I have shown a more or less conventional type which may be used for this purpose. This enlarger comprises a base or easel 50 on which a supporting structure SI is mounted. During the actual printing exposure a sheet of sensitized paper is placed on this easel. The supporting structure may be vertical or preferably slightly inclined as shown. Slidably arranged on this supporting structure is a carriage 62 which supports the projector. The main parts of this proiector are a lamp ii. a condenser 84, a film stage 55, a lens 56 and a focusing movement 51. A negative 58 can be placed on the film stage. The distance of the lens 58 from a negative It can be adjusted in the usual manner by means of the focusing movement H which may, for example, comprise a rack and pinion movement operated by a small handwhcel.

Pnorocnm. Um

Referring to Figs. 2 and 3, the photoelectric cell 20 is mounted in the conventional tube socket I20 which, in turn, is attached to a plate III. This plate is mounted in a housing I 22, the upper part of which is closed by a'cover "illthis cover having being omitted in Fig. 2; Plate I2I supports directly above the photoelectric cell a thin plate I24 with a relatively'lar'ge aperture III. Between this aperture and the photoelectric cell there is a small disc of diffusing glass I 20. It'is the purpose of this diffusing glass to prevent irregularities due to fact that without it the light impinging upon the light sensitive electrode 2i may hit more or less sensitive spots thereon. By interposing this diffusing glass theentire area of this light sensitive electrode will be evenly illuminated.

Mounted on to'pof 'plate' iii are two electromagnetically controlled diaphragms I2I'and III. Diaphragm "I28 has a medium sized aperture I28 smaller tlianthe aforementioned large aperture I25." In the example described here the area or aperture I29 is of 'thearea ofaperture I28. The diaphragm I21 has'an -aperture I3. still smaller and which in the same example has of the area of aperture I29 or i of the area of aperture I25. Each of these diaphragms has the shape shown in Fig. 2, and the two diaphragms are arranged in slightly different planes as shown in Fig. 3. Each of these plates has an upturned lug I3I and I32, respectively. and to each of these upturned lugs there is attached an iron core I33 and I34. Two electromagnetic coils I35 and I38 are provided which, when energized, attract one Of these iron cores, respectively. As shown in Fig. 2, coil I15 is represented as energized and has thereby attracted iron core in which, in turn, places diaphragm "Lin a position in which its aperture I2! is directlyabove the photo sensitive electrode ll of the photoelectric cell. In the circuit diagram, Fig. 4; both coils are shown as deeuergized. Each of the aperture plates has a slot I31 and I", respectively, and each 0! these slots engages two guide pins shown in Fig. 2. Return springs I39 and I" 4 are provided which return the diaphragm blades to their original positions as soon as sponding coil is deenergized.

MAmUNI'r the correshape containing all elements other than the photocell unit just described. These elements comprise the mechanisms to be described later by means of which the measured condenser charging times will be converted into exposure time and contrast indications, and also numerous electrical components such as a transformer. rectifying tubes, thyratrons, condensers, relays and others. The [precise location of these parts within the housingis of no consequence and they have, therefore, not been shown except on the wiring diagram of Fig. 4. -Their electrical functlon, however, will be fully explained in the following paragraphs.

n the front panel of the main unit the following elements are visible:

1. A handwheel 44' by means of which the sensitivity of the photocell can be adjusted in accordance with the sensitivity of the sensitized paper intended to be used for the print.

2. A handwheel 15' for the zero adjustment by means of which any accidental leakage within the device can be compensated for. This zero adjustment cooperates with a pointer which will appear in a window 251 and which should come to rest on a vertical line 258.

3. A push button I52 by means of which the measuring process for the brightest point on the easel can be initiated.

4. A-push button 250 by means of which the device can be reset after this measuring process.

5. A push button Hit by means of which the measuring process for the darkest point on the easel can be initiated.

6. A push button 350 by means of which the device can be reset after this process.

7. A pointer 223 indicating exposure times through a window in the front panel of the main unit on a scale 224.

8. A pointer 401 indicating contrast values through a window in the front panel of the main unit on ascale 408.

ELECTRICAL Cmcurr The electrical circuit is shown in Fig. 4. For convenience, it can be sub-divided into six parts as follows:

1. Supply circuit for multiplier tube and first condenser charging circuit.

2. Second condenser charging circuit.

3. Thyratron circuit,

4. First time-measuring unit.

5. Second time-measurin unit.

6. Push button and relay circuit.

These circuits will be described in that sequence.

Supply circuit for multiplier tube and first condenser charging circuit The multiplier tube 20 comprises a vacuated vessel, a photo emissive cathode 2|, and nine electrodes 22 to 30. The cathode 2| and the electrodes 22 to 30 are connected to corresponding talps numbered 3! to 40 of the voltage divider 4|. A condenser 42 is inserted into the second last of these connections, i. e., between points 38 and 29. The voltage divider is placed across the terminals of another condenser 43 which, in turn. receives its voltage from a potentiometer 44. This potentiometer is supplied with rectified current by means of a rectifying tube 45 and a transformer which has a secondary coil 46. an iron core 4'! and a primary coil 48. The primary coil, in turn, is connected to an alternating current line.

As can be seen, the secondary coil 46 delivers an alternating current of a suitable voltage which, by means 01' rectifying tube 45, is rectified and impressed upon potentiometer 44. Depending upon the adjustment of this potentiometer. a certain portion of this rectified voltage reaches the condenser 43 which acts as a filter and converts the rectified uni-directional alternating current into direct current with only a small rippie. This D. C. voltage is then by means of voltage divider 4| divided into ten parts. Point 2| assumes the most negative potential and is connected to the photo emissive anode of the multiplier. Going from left to right subsequent points of the voltage divider 4i become increasingly positive and each point has a positive voltage with respect to its left neighbor of approximately volts. The last point "which is com nected to the last electrode 30 of the multiplier tube is, of course, the most positive of all.

As a consequence of this arrangement, the few electrons which are released by the action of light from the cathode 2i are attracted to the nextelectrode 22 where they strike with sufficient velocity t release a number of secondary electrons. The secondary electrons are, in turn, attracted by the next electrode 23 where they release tertiary electrons and this process is repeated at each subsequent electrode. The number of the secondary electrons released at electrode 22 is larger than the number of primary electrons causing their release, and again the number of tertiary electrons released at 23 is larger than the number of secondary electrons, and the ability of the tube to multiply" electrons is based on this fact. The current circulating in the last loop, 1. e., between points 39 and 28 and between points 40 and 30 which charges condenser 42 becomes, therefore, comparatively heavy.

The light sensitivity of the tube depends upon the voltage imposed upon adjacent electrodes and. therefore, potentiometer 44, by means of which this voltage can be adjusted, provides a convenient means to adjust the sensitivity of the tube. This potentiometer can be actuated by means of a handwheel 44 shown in Fig. 1 and by means of which the sensitivity of the device can be adjusted to match the sensitivity of the sensitized paper on which subsequently a print is made.

Thesensitivity of the device can also be adjusted by changing the size of the light admitting aperture, and two electro-magnetically controlled diaphragm blades I21 and i28 serving this purpose were already shown in Figs. 2 and 3. They are again shown above the multiplier tube 20 in Fig. 4.

It can be seen that current circulating in the last loop between points 39 and 40, and 30 and 28 causes condenser 42 to be charged. Condenser 42 is, by means to be shown later, short circuited before the measuring process begins. and the time required to charge it to predetermined voltage constitutes a measure of the light impinging upon the multiplier tube 20.

Second condenser charging circuit The second condenser charging circuit is connected across the terminals of condenser 42 in parallel to the first charging circuit. Distinguished from the first charging circuit, however, the current within this second circuit is constant and entirely independent of the light impinging upon the photoelectric multiplier tube 20. The second circuit, therefore, comprises a second source of D. C. voltage and an element which keeps the current in the second charging circuit constant regardless of the voltage to which condenser 42 happens to be charged at any given instant.

The source of D. C. may be of any desired type and could, for example, be a battery. For convenience, however, I have chosen a transformer with a secondary coil II, a rectifying tube H and a condenser 72. The secondary coll ll could, of course, be associated with its own core and its own primary coil, but it can also, and this is preferable, be mounted on the same iron core 41 which was already provided for the transformer which provided current for the first charging circuit.

The current limiting device which keeps the current within the second charging current constant may be of one of several known types. For example, a screen grid tube has the property to keep the plate current constant within wide limits of plate voltages. I prefer to use as a current limiting element a second photoelectric cell II which is, in turn, illuminated by a small incandescent lamp 14 in series with a small rheostat 15. It must be emphasized that the second photoelectric cell 13 has no connection with the photoelectric multiplier tube 20 and is not exposed to the light emanating from the enlarger. This second photocell I3 is merely a convenient means of keeping the current within the second charging circuit constant.

The current circulating within the second charging circuit can be conveniently adjusted by changing the illumination of photocell 13 through manipulation of rheostat 15 which regulates the current passing incandescent lamp 1|. Rheostat 15 can be adjusted from the front of the housing ll of the main unit by means of a handwheel II.

Circuits of this type are quite sensitive to accidental leakages and even the most perfect means of insulation cannot completely eliminate any leakage. It is, furthermore, virtually impossible to keep this leakage at a constant value and it may change from day to day, depending, for example, upon temperature and humidity conditions. It is, therefore, a particular advantage of the double charging circuit as described here, that the leakage can be easily compensated for. This can be done by covering the photoelectric multiplier tube 2| so that no light from the enlarger or any other source can reach it. In other words, the first charging circuit will then be currentless except for any accidental leakage. The condenser 42 is then charged by the second charging circuit alone. The time required to charge condenser 42 by the second charging circuit alone to a predetermined voltage can then be measured and if this time differs from a standard, it can be adjusted by moving rheostat ll in one direction or theother. If, for example, the leakage within the two condenser charging circuits has increased since the last adjustment, the charging time of condenser 42 by the second charging circuit alone will usually be longer than the standard time, and in order to restore the former conditions, the charging current within the second charging circuit must be increased. This can be done by reducing the resistance of rheostat 15 so that lamp ll becomes brighter and consequently, photocell ll passes more current. An adjustment in the opposite direction, if desired, can, of course, be made in a corresponding manner.

Thyratron circuit The purpose of this circuit is to provide means to indicate when the voltage of condenser 42 has reached a predetermined critical value. It consists of a thyratron tube II, with a cathode II. a grid I2 and an anode It. This thyratron is energized by alternating current derived from a secondary coil 84 which is preferably, but not necessarily, mounted on the same iron core I! as the two other secondaries 4| and II described above. The plate circuit of the thyratron is completed by a relay coil l5 actuating a normally closed contact 92 to be described later. The grid of the thyratron is connected to the positive terminal of the condenser 42, and to complete the grid circuit the cathode ll is connected to a sliding contact 8! of a resistance I! connected across the terminal of condenser 12. It can be seen that the voltage of the thyratron grid '2 with respect to the cathode Ill consists of the voltage impressed upon the left part of resistance l1 and of the voltage impressed upon condenser 42. The two voltages are of opposite polarity. A thyratron is usually non-conductive as long as its grid voltage with respect to the cathode is more negative than 2 volts and it becomes current conducting as soon as the grid voltage is less than -2 volts negative with respect to the cathode. The result of this arrangement is that as soon as the condenser voltage is more than 2 volts larger than the voltage of the left half of resistance 81, the previously non-current conducting thyratron becomes current conducting, whereupon current begins to flow in relay coil 85.

First time-measuring unit The two time-measuring units are of identical design and comprise each a constant speed motor directly connected to the line and rotating permanently, an electromagnetic clutch and an output shaft. The output shaft is ordinarily stationary and rotates only when the clutch is energized. This takes place during the time required to charge condenser 42 and consequently the angle of rotation traveled by the output shaft is proportional to the charging time and, therefore. a function of the light impinging upon the electron multiplier tube 2l.

The first charging circuit comprises a motor ll which is preferably of the synchronous type used for clockwork movements or the like. The electromagnetic clutch Si is shown schematically as a coil in the diagram, but the actual physical appearance of the clutch is shown in Fig. 'l and will be described later. One side of the clutch is directly connected to one leg of the line, and the other side is connected across the two push button contacts, to be described later, and the normally closed contact 2 actuated by relay coil ll already described above and which is the load element of the thyratron circuit.

Second time-measuring circuit The second time-measuring circuit is identical to the first time-measuring circuit except that two rotating switch elements are mounted. on the output shaft. A constant speed motor 95, is again connected to the line and electromagnetic clutch Si is connected in the same manner as the corresponding coil SI of the first time-measuring circuit, i. e., one side of the-coil 96 is directly connected to one leg of the line, and the other side is connected across the two push buttons and the same relay contact 92.

Mounted on the output shaft are two rotating switches. the first of which short circuits condenser 42 at suitable intervals, and the second of which actuates one or the other of the solenoids I35 and I35 controlling the two diaphragm blades I23 and I21, respectively. Both switches comprise cylinders I and IN, respectively, made from insulating material, but which carry two strips I02 and I03 in the case of cylinder I00, and one strip I05 in the case of cylinder IOI made from brass or some other current conducting material.

Sliding on cylinder I00 are elastic spring contacts I05 and I01. It will be clear that when cylinder I00 rotates, strips I02 and I03 will, at certain times, conductively connect these spring contacts I06 and I01 and thereby short circuit condenser 42 at certain times. The spring contacts I05 and I01 are in series with 2. normally open relay contact I12, the function of which will be explained later.

In like manner two pairs of spring contacts I", H0 and III, II2 slide on cylinder WI and a metallic strip I05 attached to cylinder IIII will, therefore, at certain times conductively connect I00 to II! or III to H2. The contact pair I08 and H0 is in series with a normally open relay contact I and controls solenoid I35 which attracts, when energized, diaphragm blade I28. In like manner contact pair III, II2 is in series with normally open relay contact I15 and controls solenoid I36 which, when energized, attracts diaphragm blade I21.

Push button and relay circuit In order to make the operation of the device convenient for the operator and fool-proof, the entire circuit is controlled by two relays I50 and III and two push buttons I52 and I53. The push buttons are physically mounted on the front panel of the main unit and are shown in Fig. 1.

Each push button has, respectively, a normally closed contact I52 and I53 and a normally open contact I52" and I53". Relay I50 comprises an armature or coil I50, a normally closed contact I5I and two normally open contacts I62 and I63. Relay I5I comprises a coil I10, a normally closed contact HI and four normally open contacts I12, I13, I14 and I15.

The normally open contact I52" of push button I52 is in series with a normally closed contact 153' of contact I53. In like manner the normally open contact I53 of push button I53 is in series with the normally closed contact I52 of push button I52. This arrangement is a safeguard against the possibility of an ignorant operator depressing both push buttons at the same time. As it is, both push button circuits in such a case would be dead.

The normally open contact I62 of relay I50 is connected in parallel to the normally open push button contact I52". Likewise the normally open contact I13 of relay I5I is connected in parallel to the normally open contact I53" of push button I53. In other words, contacts I62 and I13 serve as hold in" contacts, and a momentary depression of push buttons I52 and I53, respectively, will energize relay coils I60 and I10 and, in turn, close all the normally open contacts of the respective relays, among them I62 and I13. Since these contacts are connected in parallel to the corresponding normally open push button contacts I52 and I53", the relay coils 10 will remain energized even after the operator releases the push buttons, and this condition will persist until the circuits will be opened at some other place, as will be described later.

The normally closed contacts I5I and HI are In series with each other and connected across the terminals of condenser 42. This condenser is thereby short circuited unless one of the relays is energized, i. e., before the start of the measuring process. Normally open contact I12 is in series with the spring contacts I06 and I01 sliding on the rotating switch element I00 which is mounted on the output shaft of the second timemeasuring unit described above. As a result of this arrangement, the rotating switch element I00 can short circuit condenser 42 only when relay I5I is energized.

Normall open contact I53 energizes, when closed, solenoid I36 which attracts diaphragm blade I21 carrying the smallest diaphragm opening for the photoelectric multiplier tube 20. The result of this arrangement is that as soon as relay I50 is energized, i. e., during the measuring process for the brightest point on the easel, the smallest diaphragm aperture I30 contained in blade I21 is placed in front of the photoelectric multiplier cell 20 and thereby the measuring process for the brightest point on the easel is always performed with the smallest light acceptance or light sensitivity of the photocell.

Normally open contacts I14 and I15 are, respectively, in series with one of the two pairs of spring contacts I09, H0 or III, II2. Due to this arrangement, rotation of switch element IOI mounted on the output shaft of motor 95 of the second time-measuring unit energizes either solenoid I36 or solenoid I35 or neither. This means that for approximately the first third of a revolution of the output shaft of the second time-measuring unit, the light acceptance of the electron multiplier tube will be governed by the diameter of the smallest aperture I30 in diaphragm blade I21, that for approximately the second third of the revolution, the light acceptance of the cell will be governed by the medium sized aperture I 29 In diaphragm blade I28, and for the last third of the revolution, this light acceptance will be governed b the large aperture I25 which is fixedly built into the top wall of the housing of the electron multiplier tube, see Fig. 3. In the interest of simplicity, no means have been shown to energize the filaments of the two rectifying tubes 45 and H and of the thyratron 80. These means may, for example, be batteries or filament transformers or, preferably, a few turns of wire may, for each tube, be wound on the core 41 of the transformer which already exists.

MECHANICAL DESIGN or FIRs'r TIME-MEASURING Unrr The actual physical appearance of the two time-measuring units can be seen in Figs. 5, 6 and 7. The first time-measuring unit is the lower one and measures the time required to charge the condenser 42 to a predetermined voltage when photocell 20 is placed on the point of brightest illumination on the easel.

It comprises a constant speed motor 90 which is preferably a synchronous motor of the type used for small clock movements or the like. These motors are commercially available with a built-in gear reduction so that the shaft I of this motor has only a slow speed; for example, a speed of 2 R. P. M. would be quite suitable. The front end of the shaft is of square or similar crosssection and two discs are carried by this shaft. The first disc ill is fixedly fastened to this shaft whereas the second disc II! which is made from ironisfreetoslldeaxiallyasmalldistance Two springs ilttendtoretractdisc ll2asmuchas possible.

The clutch coll ii is preferably surrounded by a cylindrical body of soft iron ill fastened on the output shaft I". The coil ii and the cylindrical body It! form an electromasnet which; when energized, atlu'acts iron disc m, thereby causing theentire clutch body and the output shaft 6 to rotate. Current is supplied to coil Ii by means of two flexible cables, which is possible because the clutch and the output shaft never perform more than one revolution. These flexible cables, however, are not shown in the drawings The motor N is fastened to the baseplate Ill by means of two studs Ill, and the output shaft I issupportedin abearing I". The front of the output shaft I86 carries a cam element 2, a ratchet 2", a pulley 2M and a pointer 2.5. The purpose and coaction of these elements will be described later.

As derived before the measured light intensity is ured on the brightest point or on the darkest point on the easel, and it is even conceivable to use the average value of the two. In practice, however, it is most convenient to set the exposure time in accordance with the light intensity as measured on the brightest point on the easel because the intensity of this point obviously can be measured with the best of accuracy. This simply means that a print is exposed in such a manner that well exposed shadows can be obtained and this is also in accordance with best photographic practice.

If S is the sensitivity of the photographic paper, L the light intensity measured at the brightest point on the easel and t the exposure time, we have by substituting the above value for L. we obtain:

The problem is how to use this relationship to compute the configuration of a cam which can be mounted on the output shaft of Fig. 6 of the first time-measuring unit. The radius of that cam at any given point must obviously be proportional to the exposure time, but it is also possible to add, or subtract, any desired constant. In other words, the radius" o'fthec'am' for i= does not necessarily have tobe'zero but may have some value which we shall call C3. The

12 proportionality factor between and i shall be called a, so that we have the r=t+Cs (2) 5 Since the output shaft during the measuring processrotateswithaunitormspecdtheangie s traveled by it is proportional and the proportionality factor of the two shall be called 6: so that:

w By combining Formulas l, 2 and 3 we can compute r and obtain 1 8 a C, 1 In the interest of simplicity. we call 83 so that the general equation for the cam in'a system of polar coordinates bccomu:

A development of this cam is shown in Pig. a 0n the left side the values of t and Oil-the right side the values of r are shown both as functlo'n of either p or T. Itwillbenotedanditisalsoobviousfromthe formulaforrthatrior becomes infinite and (or values of T 7' -alto This corresponds to a change of exposure tim in the ratio of 1:10 orfor example, tor" t becomes 5 seconds and for in Fig. 5. The significant points of the cam in Fig. 8 have been called Ill, 2H, 1l2',2l3', and they correspond to the points till, ii I, 2, 213 on the actual cam shown in Fig. 5. As can be seen, the portion of the cam between points 2|! and II; has a uniform radius and, therefore, does not really do any work. It would, therefore, be a waste of operating time to construct the circular cam in the same proportions and this section, therefore, has been arbitrarily shortened, as a comparison of the proportions of the circular cam of Fig. 5 to the developed cam of Fig. 8 shows.

Referring to Fig. 5 a cam-following roller 220 is in contact with the cam just described. This cam-following roller is carried by a double ended lever 22! pivotally supported by a stationary pivot 2.22 fastened to baseplate 100 and biased by a spring 125. While one end of lever 22l carries the roller 220, the other end carries pointer 223. The tip of this pointer is visible through a window in the front panel of the main unit, see Fig. 1,'

In logarithmic terms this equation reads i i Log L=log i-log T l The radius R of this cam is proportional to the logarithmic value of L, and again an arbitrary constant may be added thereto since R does not necessarily have to become zero when log L becomes zero. We have therefore where b and C5 are constants. Substituting the above value of log L:

1 R=b+c. log -+c.1o (f-1) The angle o of the cam, in a system of polar coordinates, is again proportional to o the proportionality factor being called Cs, and for convenience, the tenns b+c. log

can be lumped together to form a new constant C4. The equation of the cam in polar coordinates therefore reads:

1 .RC4+Cs log (K -1) A development of this cam is shown in Fig. 9.

0n the left side we have log L, and on the right side R, both as functions of For log L becomes infinite and for T -1 log L becomes negatively infinite. The usable portion of that cam must, therefore, not approach the zero value or the unity value of o too closely. As a convenient value, I have chosen to start with and extend the cam to which means that log L varies between .954 and .046; in other words, the difference between is 10 times as large as the linear light intensity for As can be seen in Fig. 9, the initial value of R for has been chosen rather large and the useful area of the cam in the developed presentation in Fig. 9 has again been shaded for convenience. The actual cam can be seen in Fig. 5. The significant points of the developed cam in Fig. 9 have been called 230', 23!. 232' and 233'. They correspondto points 230, 21H, 232 and 233 on the actual circular cam shown in Fig. 5. Again, the radius of the cam between points 232 and 233 is constant so that no real work is performed in this portion. This part has, therefore, been arbitrarily shortened in order to save operating time, as a comparison between the circular cam illustrated in Fig. 5 and the developed cam 11- lustrated in Fig. 9 will show.

While it would, of course, be entirely feasible and practical to have the two cams individually and separately mounted in two planes on shaft ill, the actual construction can be somewhat simplified due to the fact that either cam occupies less than This makes it possible to arrange both cams in one plane so that the two apparently constitute one single cam which is shown in Fig. 5. The left half of this cam performs the function of converting the measured condenser charging time into paper exposure time, whereas the right half of this cam performs the function of converting the same condenser charging time into logarithmic light intensity values. The two parts are connected by lines Iii-430 and 2i0233 which may be of any convenient shape.

Since, during operation of the device, i. e., during the measuring process, the output shaft I88 with all the elements connected thereto moves from the zero position into some position indicatlve of the measured light value, it becomes necessary to provide means to keep these elements in the extreme position which they occupy after the light measuring process has been terminated by action of thyratron I. and'the relays controlled thereby. Likewise means must be providedtoreturnallthesepartstotheiroriginal starting position at the option of the operator before a new measuring process can be started. Referringtofig-fiaratchetgcarfll andapulley 2 have already been described which are fastenedtotheoutputshattiflofthefirsttimemeasuringunit. Attachedtopulleyalisa spring 2 which biases all elements attached to shaft I" in a clockwise direction Cooperating with ratchet gear 261 is a ratchet 2 which is under the influence of a spring 2 and which prevents thereby ratchet gear 261 from returning to its starting position in a clockwise direction, which it is urged to do by the tension of spring 246.

Fastened to ratchet 2 is a small bracket 2 which carries a roller 246. A push button 256 is mounted on the front panel of the main unit which is pressed by aspring 2H and which carries a tapered part 252. This tapered part is in contact with roller 2&6.

Durin the charging time of the condenser the clutch is energized and, therefore, shaft I86 with ratchet gear 202 rotates in a counter-clockwise direction. Assoonascondenserflhasreached a predetermined voltage the clutch is deenerg ed and, therefore, shaft I86 ceases to rotate. Spring 2 urges the shaft and the elements connected withittoreturninaclockwisedirection intotheir starting position, but as long as ratchet 2 is in operative engagement with ratchet gear 263 this is rendered impossible. In other words the ratchet gear 202 is arrested in its extreme counterclockwise position. The two cams on the circumference of element 262 rotate, of course, by the same angle and affect, therefore, the position of pointer 223 as well as the position of a differential mechanism for the indication of the contrast which will be describai later.

When the operator chooses to reset the first time-measuring device because the indicated measurement is no longer wanted, or because another measurement is desired. he depresses push button 2". Push button 256 with the associated tapered part 252. Fig. 6, moves from left to right, causing roller 246 to perform a corresponding vertical movement. This, in turn, causes ratchet I to swivel slightly around its supporting pivot.

' bringing its tip out of engagement with ratchet wheel 2". whereupon the pull of sprin 2 can exert itself and return the ratchet wheel with the cam 20: and shaft I86 into its starting position. This starting position is determined by a pin 255 mounted on a base-plate 209.

A pointer 205, shown in Fig. 7, was already described and is carried on the extreme front end of shaft I86. This pointer is used to determine the proper leakage conditions of the device. The Operation of this leakage indicator will e described later.

Mzcmnnclu. Dasron or Sitcom: Tnm-Mmsnanvo Um The design of this unit is in many respects identical with the design of the corresponding first unit. It comprises a constant speed motor IS, a clutch 98 and an output shaft 286. Mounted on this output shaft is a cam 362, a ratchet gear I" and a pulley I", see Fig. 7. Fastened 16 topulleyulisasprin lll which biasesthe entire output shaft assembly in a clockwise direction. with ratchet gear 381 is a ratchet III associated with a push button controlled resetting device in precisely the same manner as in the first time-measuring unit, The corresponding push button 360 can be seen in ig. 1.

The second time-measurin unit dlifers from the first time-measuring unit in two respects: Two rotating switch elements I" and Iii are mounted on the ouiput shaft 286, and a diil'ermt cam is provided.

The two rotating switch elements I" and III @Optfflte'llthflJlihgOODtBdSShOWhlllPlLi. The electrical fimction of this arrangement has already been explained,

Thepurposeofthecamassociatedwith the second time-measuring unit is to give an indication of the logarithmic light intensity value of the darkestpointmeasuredontheeasel. Thisis in some respects more diiliwlt than the corresponding indication of l garithmic light intensity value for the brightest point on the easel because here the range could bellmited to a ratio of 1:10. In view of the fact that the operator can adjust the light output of the enlarger by means of the diaphragm or by other meam within fairly wide limits, this limitation appears reasonable, but a similar limitation cannot be imposed upon the range in which the darkest point may fall. The reason for this is obvious. Not only may the light intensity of the brightst point vary in the proportion of 1:10, but the contrast of the negative itself may vary within the range of approximately 1:100. These two ranges together determlnetheexp ctcdrangeofthellghtintensity for the darkest point which may, therefore, be of the general order of 1:1000. In order to accommodate this exceedingly wide range the followin means are employed:

The second time-measuring unit is made to run through an operating cycle substantially identical to the operating cycle of the first timemeasurin unit, but not once but three times. Between these operating cycles the wndenser is automatically discharged since it is short circulted by the action of switch element I00, see Fig. 4, mounted on the output shaft 286. During the first operating cycle the smallest apertlire I30 is placed in front of the photoelectric multiplier tube 20. During the second operating cycle the next larger diaphragm opening I28. is in this position, and for the third operating cycle the light admittance of the cell is controlled by the aperture I25 which is still larger and which is in the top wall of the photocell housing I2I. This change of apertures is eifected by the action of the switch element IOI which, during the first cycle, energizes solenoid I16, which energizes solenoid I36 during the second cycle, and which renders both solenoids currentless durin the third cycle. As a result of this change of apertures, the light sensitivity of the device is systematically increased from cycle to cycle, and if condenser 42 fails to reach the predetermined voltage at which it actuates thyratron 80 and stops motor during the first cycle, because, due to the small aperture, the photocell current is too small. it may do so during the following or the next following cycle, when the photocell current, due to the larger aperture, is correspondingly higher.

It is understood that in reality the second time-measuring unit will run through all three cycles only if the negative to be measured is of 17 rather extreme contrast. For ordinary negatives, motor will come to a standstill much earlier, usually during the second cycle, and in the case of very flat negatives, motor may even stop during the first operating cylcle.

The design of cam 302 is based on this triple cycle, and a developed representation of it is shown in Fig. 10. It can be seen that the effective circumference of the cam has three divisions. each of which are substantially identical to the configuration of the cam shown in Fig. 9. The three branches of the curve are. of course, radially displaced with respect to each other in such a way that the maximum radius of one portion is identical with the minimum radius of the adjacent portion. The formulae for these branches are precisely identical to the formula for the cam curve in Fig. 0 with the exception, of course, that diiferent constants may be chosen:

R =C +C log a-1) 2 l0+ ll 8 (6%,

a= n+ u 8 37;

The significant points of the developed cam of Fig. II are 360', 36i, 302', 363', 364, 305', 300' and 301'which correspond to points 380 to 361 of the circular cam in Fig. 5. Again, certain parts with a constant radius have been arbitrarily shortened to save operating time. In the development of this cam shown in Fig. 10, I have also illustrated in the interest of clarity the function of the two circuit elements I00 and IN to show the angular relations between the efiective parts of these elements and the three branches of the cam.

Corrrrusr Imarcaron The contrast indicator is a mechanical movement adapted to indicate the difference between the two logarithmic light values, which is the definition of photographic contrast, or in mechanical terms the difference between the radius of the logarithmic part of cam 202 and the radius of cam 302. While a great number of mechanisms are conceivable which would serve this purpose, I prefer the following construction which is exceedingly simple:

A pivoted lever 400 carries two cam-following rollers 40f and 402 which are in contact with cams 202 and 002, respectively. The pivot point of this lever is 404 which is carried by a second pivoted lever 400. This last lever is supported by a stationary pivot 400 fastened to baseplate 200, and carries on its other end a pointer 401. This pointer is visible through a window of the front panel of the main unit and indicates contrast values on a scale 400, see Fig. 1. A spring 410 biases lever 400 in a clockwise direction. Due to this bias the two cam-following rollers 401 and 402 are always in contact with cams 202 and 302, respectively.

The function of this device is quite simple. Assuming that only cam 202 moves and cam 302 remains stationary, any increase or decrease in the radius of cam 202 will cause lever 400 to swivel around the contact point between cam 302 and roller 402. It can be easily seen that pivot point 404 then performs a movement equal to half the increase or decrease of the radius of cam 202. In like manner, assuming that cam 202 remains stationary and cam 302 rotates, lever 400 performs a rotary movement around the contact point of roller 40l and cam 202, and the pivot point 404 participates in this movement to the extent that it is displaced by half of the displacement of roller 402 or, which is the same, by half the increase or decrease of the radius of cam 302.

It can be seen that an increase of the radius of cam 202 as well as an increase in the cam 302 will cause lever 400 to rotate in a. counter-clockwise direction. If the two increments are the same, pivot point 404 will remain stationary, and consequently lever 405 with pointer 401 will also remain stationary. If, however, the increase of the radius of cam 20! is different from the increase of the radius of cam 302, pivot point 404 will be displaced and this displacement will equal half the difference of the two radial increases. or the displacement of pivot point 404 and therewith the movement of lever 405 and pointer 401 will be in proportion to the difference of the two radii or of the two logarithmic light intensities. In other words, pointer 401 will indicate on scale 408 the contrast of the image projected on the easel.

The angles of the arcs described by rollers MI and 402, and by point 404, must, of course, be reasonably small, so that the arcuate paths of these elements do not deviate appreciably from straight lines.

Ormrron From the operator's viewpoint the operation of the device is exceedingly simple. The operator first ascertains whether the pointers 223 and 401 are at the lowest points of scales 224 and 408, respectively. If they are not, he depresses push buttons 250 and 200 and causes thereby the two indicator to return to their starting points. The operator then tests the leakage conditions of the device by covering the photocell and depressing button I02, the on button for the brightest point on the easel. After a short interval of time, pointer 205 should then appear in window 201 and come to a standstill in-register with line 200. If the pointer 205 overshoots this line or comes to a standstill before it reaches it the device has either more or las leakage than expected and this can be changed by adjusting knob 10 in one direction or the other. The device should then be reset by depressing button 200 and tried against by depressing push button I02 until pointer 205 comes to a standstill on line 200. The device is then reset by depressing push button 260 again. Handwheel 44' must, before the final operation of the device, be adjusted for the sensitivity of the sensitized paper which the operator expects to use for the print. The photocell is then uncovered and placed on the easel of the enlarger in which a negative has been placed and which has been adiusted for the desired magnification ratio and diaphragm setting of the lens. The photocell is then shifted on this easel until the brightest point on the image falls on the window in the top wall of the photocell housing. Pushbutton I52 is then depressed and the operator waits until the movement initiated thereby is finished. The photocell is then shifted until the darkest point on the easel is projected into the window. Push button I53 is now depressed and when the movement induced thereby is finished, pointers 223 and 401 indicate exposure time and contrast, respectively. What happens within the device will be described in the following paragraphs:

When the operator depresses push buttons 250 and 350 for resetting purposes. he lifts the tips of ratchets 24I and I so as to bring them out of engagement with the ratchet gears 203 and 303. see Figs. 5 and 6. In the case of push button 250 this is done by the action of the tapered part 252 which by its horizontal movement from left to right forces roller 246 upward. This roller is supported by a bracket 245 which, in turn, is fastened to the ratchet 2. Push button 350 is, of course, associated with a corresponding mechanism which actuates ratchet 34L As soon as the two ratchets lose contact with ratchet gears 203 and 303, springs 240 and 340 which are fastened to pulleys 204 and 304 cause these pulleys together with all elements mounted on the respective output shafts I86 and 206 of the first and second time-measuring unit to rotate in a clockwise direction until they are stopped by stop pins 255 and 355, i. e., until all these elements assume the position shown in Fig. 5.

For the test of the'leakage conditions of the device the photoelectric multiplier cell 20 should be covered, i. e., no current should circulate between the taps of the potentiometer 4i and the corresponding electrodes of the multiplier tube 20 except such leakage currents as are inevitable due to the impossibility of providing perfect insulation. The condenser 42 is then charged solely by the current supplied by the second charging circuit, i. e., by the current passing from condenser I2 through the current limiting photocell I3. Condenser I2, in turn, is charged from the transformer secondary coil I through rectifying tube I I, and the current passing current limiting photocell I3 depends upon the illumination which this photocell receives from the small incandescent lamp I4. This current can be adjusted by rheostat I5 actuated by handwheel Any adjustment of rhecstat I5 will increase or decrease the light of incandescent lamp I4 and thereby increase or decrease the current circulating within the second charging circuit of the condenser, and this will charge condenser 42 faster or slower. If the charging time of the condenser 42 should be difierent from a standard time the leakage somewhere in the system has obviously changed, and the original time required to charge condenser 42 to a predetermined voltage by the second charging circuit alone can be restored by adiusting rheostat I5 in one direction or the other.

The sensitivity of the photocell 20 can be adjusted by adjusting potentiometer 44. Mechanically this is done by manipulating handwheel 44. The adjustment of potentiometer 44 changes the voltage impressed upon adjacent electrodes of the photo multiplier tube, and this is a very effective means for adiusting the sensitivity of this cell to light. This adjustment is used to change the light sensitivity of the photocell in proportion to the lightisensitivity of the sensitized bromide paper which the operator has selected for the future print.

For the final operation of the device, it is immaterial whether the brightest or the darkest point on the easel is measured first, and merely by way of example I shall describe the measuring process for the brightest point first. Before the start of this measuring process, push button 250 must, of course. be pressed again so that the device starts from the zero setting. Referring to the circuit diagram shown in Fig. 4 it can be seen that condenser 42 before the beginning of the measuring process is short circuited by normally closed relay contacts IBI and I'll connected in series with each other. The two motors 00 and 95 are permanently connected to the line and rotate continuously, but since the clutches 9| and 98 are deenergized the output shafts I06 and 200 of the two time-measuring units are stationary. The operator now depresses push button I62. This closes the normally open push button contact I52" and closes a circuit which is in series with the normally closed push button contact I53 of the second push button I53, and with the normally closed contact 02 of relay 05, thereby energizing relay coil I00 and clutch coll 0|. Clutch col-l 0| causes magnet I05 to attract iron disc I02 whereupon output shaft I05 with all the elements associated with it begins to rotate in a counter-clockwise direction, see Figs. '1 and 5. As soon as relay coil I becomes energized, normally open relay contact I02 closes, normally closed relay contact IGI opens and normally open relay contact I83 closes. I62 is connected in parallel to I52" therefore constitutes a hold-in" contact keeping relay coil I60 energized even after the operator ceases to depress push button I52. The opening of IBI interrupts the short circuit or condenser 42 which now begins to accumulate a charge by the combined actions of the first and second charging circuit. The closing of contact I63 causes current to a pass solenoid I36 attracting diaphragm I21 and placing the smallest aperture I30 in front of the photosensitive electrode 2i of the photoelectric multiplier tube 20. The light striking this photosensitive electrode causes a relatively small number of electrons to strike the next electrode 22 where they release secondary electrodes which, in turn, strike electrode 23 and release, therefore, tertiary electrons, and this is repeated at each subsequent electrode. The electrons released at eachelectrode are more numerous than the electrons causing their release and, consequently, a relatively heavy current begins to circulate in the last loop of the network connecting the taps of potentiometer 4| to corresponding electrodes of tube 20. This loop is formed by the connections between points 39 and 40 to electrodes 20 and 30, respectively. The consequence of this circulating current is that condenser 42 accumulates a charge, its upper terminal becoming more and more positive and its lower terminal becoming more and more negative. The upper terminal is connected to the grid 02 of thyratron 00 and the lower terminal is connected in series with the left portion of potentiometer 81 to the cathode 8I of this thyratron. Condenser 42 and the left portion of potentiometer 01 are of opposite polarity, and at the beginning of the measuring circuit the grid of the thyratron is, therefore, highly negative with respect to the cathode. This negative bias diminishes during the measuring process and, as soon as the difference becomes small enough, the thyratron which, due to the negative bias has not conducted any current up to now, breaks down and becomes current conducting. This usually takes place when the grid becomes less than 2 volts negative with respect to the cathode. The time required to charge condenser 42 up to the critical voltage necessary for this condition thereby constitutes a measure of the intensity of the light impinging upon the photosensitive electrode 2| of the multiplier tube 20. As soon as the thyratron 80 begins to conduct .current, alternating current originating in secondary transformer coil 84 energires relay coil 85, thereby opening the norma-lh; closed contact 02. The opening of this contact 82 interrupts the current which kept relay coil I and clutch II energized, whereupon relay contacts I02 and I03 become open and contact I6I becomes closed again. This last action short circuits and thereby discharges condenser 42 which is now ready for the next measuring process. During the time required to charge condenser 42 to its critical voltage, output shaft I86 of the first time-measuring unit, Fig. 7, keeps rotating due to the action of clutch 0| connecting output shaft III to the shaft III of motor 00. The angle of this rotation is proportional to the time required to charge the condenser and is thereby a measure of the light intensity of the brightest point of the easel. Cam 202 participates in this rotation and causes a corresponding movement of camfollowing roller 220 and lever 22I with its pointer 222 indicating the required exposure time on scale 224. following roller 40I and a movement of levers 400 and 400. The position that these last two parts assume after the end of the first timemeasuring process is, however, of no significance so long as the intensity of the darkest point of the easel has not yet been measured.

The intensity of the darkest point on the easel is measured by shifting the photocell 20 with its housing until this darkest point is projected into the window of this housing. After push button I" is depressed, another condenser charging and time-measuring process is initiated which electrically is identical to the process applied to the brightest point on the easel just described. However, due to the fact that the light intensity of the darkest point of the easel may possibly change within much wider limits than the light intensity of the brightest point, provisions have been made to cause the second time-measuring device to run through its operating cycle not once, as the first time-measuring unit, but three times. Progressively larger apertures are placed in front of the photocell for the three subsequent cycles, and the condenser 42 is automatically discharged between cycles. For the following detailed description it may be presupposed that we are dealing with an extremely contrasty negative. What happens in this case is as follows:

As soon as the operator depresses push button I02, he closes the normally open push button contact I02" and closes a circuit which is in series with the normally closed push button contact I02 of the first push button I52 and with the normally closed contact 92 of relay coil 85, thereby energizing relay coil I10 and clutch coil 90. Energizing clutch coil 96 causes the output shaft 200 with all the elements associated with it to rotate in a counter-clockwise direction, see Figs. and '1. As soon as relay coil I becomes energized, normally open relay contact I13 closes, normally closed contact I'II opens, normally open contact I12 closes and the two normally open contacts "4 and I10 close. I13 is connected in parallel to I03" and therefore constitutes a hold-in" contact keeping relay coil I10 energized even after the operator ceases to depress push button I02. The opening of III interrupts the short circuit of condenser 42 which now again begins to accumulate a charge. The closing of contact I12 makes it possible for the current conducting strips I02 and I03 on rotating switch element I00 to connect spring contacts I00 and Ill and thereby to discharge condenser 42. This, however, takes place only between the first and second, and between the second and third op- It also causes a movement of camerating cycle. In like manner the closing of contacts I14 and I" makes it possible for the current conducting strip I00 on rotating switch element III to short either spring contacts III, H2 or spring contacts I00, I I0. The first pair is shorted during the first operating cycle thereby energizing solenoid I20 and placing the smallest aperture I20 in front of the light sensitive electrode 2I of photocell 20. The second pair of spring contacts I00, H0 is shorted during the second operating cycle thereby energizing solenoid I" which places the next larger aperture I 20 in front of the photocell 20. During the third operating cycle neither pair of spring contacts is connected and both solenoids I30, I20 are, therefore our-- rentless during this period. Consequently, the light admittance of the photocell is then controlled entirely by the largest aperture I2l built into the housing of the photocell.

Condenser 42 is again charged in precisely the same manner as described above by the combined action of the first and of the second charging circuit and again renders thyratron 00 current conducting as soon as it accumulates enough charge to render the grid of this thyratron less than -2 volts negative with respect to the anode. We have assumed that we are dealing with an extremely constrasty negative. In this case the darkest point will receive so little light that condenser 42 does not reach the critical voltage during the first operating cycle or, in mechanical terms, points 362 of the cam circumference will pass cam-following roller 402 and still the condenser will not have reached a voltage high enough to bring the device to a standstill by the action of thyratron 00 and relay coil 00 with its normally closed contact 02. The consequence is that motor keeps on rotating. The rotating switchelement I00 then discharges the condenser 42 and opens the circuit again so that the second operating cycle begins and condenser 42 can accumulate a charge again. Since we assumed that the negative is extremely contrasty and the darkest point, therefore, receives very little light,

the condenser 42 will still not reach the critical voltage during the second operating cycle even though the light admittance of the photocell is now ten times larger due to the larger area of aperture I20 now in front of the photocell because rotating switch element IOI now energizes solenoid I00 thereby attracting diaphragm blade I20. -In other words, point 204 on the circumference of the cam will also pass cam-following roller 402 without the condenser having reached the critical voltage where thyratron 00 becomes current conducting. Again rotating switch element I00 shortcircuits condenser 42 and rotating switch element I 0| now renders both solenoids I20 and IN currentlass. The light admittance of the photocell is now governed by the large aperture I20 in the top wall of the photocell housing. During the third operating cycle, 1. e., after point 200 of the coil 202 has passed cam-following roller 402, condenser 42 will again become charged, but this time at a still higher rate of speed due to the now times higher light admittance of the largest aperture I25. Eventually when some point between points 260 and 200 of cam 202 passes cam-following roller 402 the condenser 42 will have a voltage sufilciently high to render grid 82 less than --2 volts negative with respect to the cathode II of thyratron 00, whereupon this thyratron becomes current conducting and causes relay coil 00 to open normally closed relay contact 02. The opening of this contact '23 in turn opens relay coil I and the clutch coil 06, thereby bringing the entire measuring process to an end, and output shaft 206 with its associated parts to a standstill.

For less contrasty negatives the critical point may be reached long before the second time-measuring unit enters its third operating cycle. The device may, for example, come to a standstill sometime during the second operating cycle, i. e., when cam-following roller 40! is in contact with some point between points 363 and 364. For negatives of very low contrast this may even take place during the first operating cycle, i. e., when roller 402 is in contact with some point of the cam between points Ni and 362.

It has been explained above that the mechanical movement which comprises the two cam-following rollers 40! and 402, first pivoted lever 4M and second pivoted lever 405 with its pointer 4'01, assumes a position which is indicative of the difierence between the radii of the respective cams 202 and 302 or proportional to the diiference oi the logarithmic light intensity values of the brightest and darkest point On the easel. The position of pointer 401, therefore, indicates on scale 408 the contrast of the image.

It is obvious that various changes of form, proportion, minor details and combination of parts may be resorted to without departing or sacrificing any of the advantages of the invention, as defined in the appended claims.

What I claim as new, is:

l. A print control instrument for photographic enlargers which include a source of light and project an image of a photographic negative on a sheet of sensitized material placed on an easel, comprising a photoelectric cell, adapted to be placed successively on the two points of brightest and darkest light intensity on said easel, respectively, an electrical circuit operatively connected to said cell and including a condenser adapted to change its voltage upon exposure of said photoelectric cell to light, a first and a second measuring device adapted to measure the time required by said condenser to change its voltage by a predetermined magnitude when said photoelectric cell is placed on the brightest and darkest point on the easel, respectively, each of said measuring devices including an output shaft adapted to rotate with a constant speed during the time required for said voltage change, mechanism operatively associated with the output shaft of said first measuring device converting its rotary travel into a movement inversely proportional to the light intensity of the brighest point of the easel and an element actuated by said mechanism indicating exposure time, means also operatively associated with the output shaft or said first measuring device converting its rotary travel into a movement proportional vto the logarithmic value of the light intensity of the brightest point on the easel, means operatively associated with the output shaft of 'said second measuring device converting its rotary travel into a movement proportional to the logarithmic value of the light intensity of the darkest point on the easel, and mechanism operatively associated with said two last named means indicating the difference of two logarithmic light intensities and an element actuated by said last named mechanism indicating contrast.

2. A print control instrument according to claim 1, having a first and a second parallel charging circuit operatively connected to said condenser, said condenser adapted to be charged by them simultaneously, said first charging circuit comprising a first source of voltage and said photoelectric cell, adapted to be placed substantially into the same plane as said sensitized sheet during the actual exposure, said first source of voltage, said photoelectric cell and said condenser connected in series,,whereby said condenser, upon exposure 01 said photoelectric cell to light, is charged with a rate of speed depending upon the intensity of said light, said second charging circuit comprising a second source of voltage and means to limit the current of said second charging circuit to a substantially constant value, said second source of voltage, said means to limit the current, and said condenser connected in series, whereby said condenser is charged with a constant rate of speed, independent of the intensity of said light.

3. A print control instrument according to claim 1, having a first and a second parallel charging circuit operatively connected to said condenser, said condenser adapted to be charged by them simultaneously, said first char ging circuit comprising a first source of voltage and a photoelectric cell, adapted to be placed substantially into the same plane as said sensitized sheet during the actual exposure, said photoelectric cell being of the multiplier type including a photo emlssive cathode and n anodes, said source of voltage including a voltage divider with a starting tap and 11 additional taps, the starting tap being of negative polarity and connected to said cathode, the other taps being progressively more positive and connected to corresponding anodes of the multiplier tube, and said condenser being inserted into the connection between the (n-l)st tap and the (n1)st anode, whereby said condenser, upon exposure oi said photoelectric cell to light, is being charged with a rate of speed in dependence of the intensity of said light, said second charging circuit comprising a second source of voltage and means to limit the current of said second circuit to a substantially constant value, said second source of voltage, said means to limit the current, and said condenser being'connected in series, whereby said condenser is being charged with a constant rate of speed, independent of the intensity of said light.

4. A print control instrument according 'to claim 1, having a first and a second parallel charging circuit and operatively connected to said condenser, said condenser adapted to be charged by them simultaneously, said first charging circuit comprising a first source of voltage and said photoelectric cell, adapted to be placed substantially into the same plane as said sensitized sheet during the actual exposure,-said first source 01' voltage, said photoelectric cell and said condenser connected in series, whereby said condenser, upon exposure of said photoelectric cell to light, is charged with a rate of speed depending upon the intensity of said light, said second chargin circuit comprising a second source of voltage and means to limit the current of said second charging circuit to a substantially constant value, said second source of voltage, said means to limit the current, and said condenser connected in series, whereby said condenser is charged with a constant rate 01' speed, independent of the intensity of said light, and means to adjust said'current limiting means until, without any light impinging on said photoelectric cell, said condenser is charged by said second charging circuit alone to a predetermined voltage in a predetermined time, whereby any accidental leakage in the circuit of said condenser can be substantially compensated for.

5. A print control instrument according to claim 1, having a first and a second parallel charging circuit operatively connected to said condenser, said condenser adapted to be charged by them simultaneously. said first charging circuit com-prising a first source oi! voltage and a first photoelectric cell, adapted to be placed substantially into the same plane as said sensitized sheet during the actual exposure, said first source of voltage, said first photoelectric cell, and said condenser connected in series whereby said condenser, upon exposure of said photoelectric cell to light, is charged with a rate of speed in dependence of the intensity of said light, said second charging circuit comprising a second source oi voltage and means to limit the current 01' said second circuit to a substantially constant value, said second source of voltage, said means to limit the current, and said condenser connected in series, whereby said condenser is charged with a constant rate of speed, independent of the intensity oi said first mentioned source of light, said means to limit the current comprising a second photoelectric cell and means to illuminate said second photoelectric cell with a constant intensity, independent of the intensity of said first mentioned source of light within said enlarger.

6. A print control instrument according to claim 1, having a first and a second parallel charging circuit operatively connected to said condenser, said condenser adapted to be charged by them simultaneously, said first charging circuit comprising a first source of voltage and a first photoelectric cell, adapted to be placed substantially into the same plane as said sensitized sheet during the actual exposure, said first source of voltage, said first photoelectric cell, and said condenser connected in series whereby said condenser, upon expcsure of said photoelectric cell to light, is charged with a rate of speed in dependence of the intensity 01' said light, said second charging circuit comprising a second source oi voltage and means to limit the current of said second circuit to a substantially constant value, said second source of voltage, said means to limit the current, and said condenser connected in series, whereby said condenser is charged with a constant rate of speed, independent of the intensity of said first mentioned source of light, said means to limit the current comprising a second photoelectric cell and means to illuminate said second photoelectric cell with an intensity independent of the intensity of said first mentioned source of light within said enlarger, and means to adjust said last named means to illuminate to adjust the current of said second charging circuit, until said condenser, without any light impinging upon said first photoelectric cell, becomes charged by said second charging circuit alone to a predetermined voltage in a predetermined time, whereby any accidental leakage in the circuit of said condenser can be substantially compensated for.

7. A print control instrument according to claim 1, said photoelectric cell being of the multiplier type including a photo emissive cathode and a plurality o! anodes, said circuit associated with said cell comprising a source of direct current and a voltage divider connected thereto with a starting tap and a plurality oi additional taps, the number of additional taps being equal to the number or anodes, the starting tap being 0! negative polarity and connected to said cathode, the other taps being progressively more positive and connected to corresponding anodes of the multiplier tube, and means to adjust the voltage of said direct current whereby the light sensitivity of said photoelectric cell can be changed in accordance with the light sensitivity of said sensitized sheet,

8. A print control instrument according to claim 1, said electric circuit comprising means to short circuit said condenser before the start of the measuring process, a source of current in series with said condenser and said photoelectric cell, means to render said means to short circuit inoperable alter the start of the measuring process, whereby said condenser accumulates a charge which increases during the measuring process at a rate of speed in accordance with the intensity of the light impinging upon said photoelectric cell, and means to indicate when said charge equals a voltage of a predetermined magnitude.

9. A print control instrument according to claim 1, said electric circuit comprising means to shortcircuit said condenser before the start of the measuring process, a first source of direct current in series with said condenser and said photoelectric cell, means to render said means to shortcircuit inoperable after the start of the measuring process, whereby said condenser accumulates a charge which increases during the measuring process at a rate of speed in accordance with the intensity of the light impinging upon said photoelectric cell, and means to indicate when said charge equals a voltage of a predetermined magnitude, including a thyratron with a grid circuit and an anode circuit, said grid circuit comprising a second source of direct current, the positive terminal of said second source connected to the cathode of the thyratron, the negative terminal of said second source connected to the negative terminal of said condenser, and the positive terminal of said condenser oonnected to the grid of the thyratron, whereby the voltage 01 said grid with respect to the cathode of said thyratron becomes equal to the difference of the voltages of said second source of direct current and of said condenser, so that the thyratron is rendered nonoonducting before and during the light measuring process as long as said condenser voltage is small and the grid therefore highly negatively biased with respect to the cathode, but becomes current conductive at the end of the measuring process as soon as said voltage diiference, due to the rising condenser voltage, falls below a critical value, and said anode circuit including a source of alternating current and a load device, in series connection with said thyratron and said source of alternating current, and actuated by the anode current of said thyratron at the end of the measuring process.

10. A print control instrument according to claim 1, said electric circuit comprising means to shortcircuit said condenser before the start of the measuring process, a source of current in series with said condenser and said photoelectric cell, means to render said means to shortcircuit inoperable after the start of the measuring process, whereby said condenser accumulates a charge which increases during the measuring process at a rate of speed in accordance with the intensity of the light impinging upon said photoelectric cell, means to indicate when said charge equals a voltage of a predetermined magnitude. each of said two time-measuring devices comprising a constant speed motor, an eletic clutch. an output shaft. means to keep said clutch dcenergized before the measuring process, and means operativeiy connected to said means to indicate to energize said clutch during and deenergize it at the end of the measuring process. whereby said output shalt rotates during the charging time of said condenser, the angle oi rotation being proportional to said charging time of said condenser and thereby in accordance with light intensity to be measured.

11. A print control instrument according to claim Lsaidelectriccircniteomprisingmeans to sbortcircuit said condenser bdore the start of the process, a first source of direct current in series with said condenser and said photoeiecticcelmeanstorendersaid meansto shortcircuitinoperableafterthestart ofthe measuring proces, whereby said condenser accmnulatesachargewhichincreasesduring the measuringprocessatarateofspeedinaccordance with the intensity of the light impinging uponsaidphotoelectriccelLmeanstoindicate when said charge equals a voltage of a predetermined magnitude, including a thyratron with a grid circuit and an anode circuit, said grid circuit comprisin a second source of direct currcnt,theposit.iveterminaloisaidsecondsource connectedtothecathodeofthethyratron, the negative terminal of said second source connected to the negative terminal said condenser, and the positive terminal of said condenser connected to the grid of the thyratron, whereb said thyratron is rendered nonconducting before and during the measuring process, but becomes conductive at the end of said process, said anode circuit including a source of alternating current and a relay with an armature and contacts, said armature in series connection with said thyratron and said source of alternating current. and actuated by the anode current of said thyratron at the end-of the-measuring process, each of said two time-measurin devices comprising a constant speed motor, an electromagnetic clutch, an output shaft, means to keep said clutch deenergized before the measuring process, and means operatively connected to said relay contacts to energize said clutch during and deenergize it at the end of the measuring process, whereby said output shaft rotates during the charging time of said condenser, the angle of rotation being proportional to said charging time of said condenser and thereby in accordance with the light intensity To be measured.

12. A print control instrument according to claim 1, said electric circuit comprising means to shortcircuit said condenser before the start of the measuring process, a source of current in series with said condenser and said photoelectric cell, means to render said means to shortcircuit inoperable after the start of the measuring process, whereby said condenser accumulates a charge which increases during the measuring process at a rate of speed in accordance with the intensity of the light impinging upon said photoelectric cell, means to indicate when said charge equals a voltage of a predetermined magnitude, each of said two time-measuring devices comprising a constant speed motor, an electromagnetic clutch. an output shaft, means to keep said clutch deenergized before the measuring process, and means operatively connected to said means to indicate to energize said clutch during and deenergize it at the end of the measuring process. and means urging said shaft to return VJ l d 28 starting position, means maintaining said shai aftertheendofthe mcasurlngprocesslnits most advanced position against said urging means, and means under control of the operator to render said last named means inoperative for resetting purposes.

13. A print control instrument according to claim 1, said electric circuit comprising means toshortcircuitsaid condenserbeiorethe startof the measuring process, a first source or direct wrrentmserieswithsaidcondenserandsaid photoelectric cell, means to render said means to shottch'cuit inoperable after the start of the measuring process, whereby said condenser accumulates a charge which increases during the measuringproceaatarateofspeedinaooordance with the intensity of the light impinging upon said photoelectric cell, means to indicate whensaid chargeequals avoltageot apredetermincd magnitude, including a thyratron with a gridcircmtandananodecircuihsaidgridcircuitcomprisingasecondsom-ceotdirectcxnrent,thepomtiveterminalofsaidsecondsourcc connectedtothecathodeofthethyratronthe negative terminal of said second source connected to the negative terminal of said condenser. and the positive terminal of said condenser connected to the grid of the thyratron, whereby said thyratron is rendered nonconducting before and during the measuring process, but becomes conductive at the end of said process, said anode circuit including a source of alternating current and a relaywith an armature and contacts, said armature in series connected with said thyratron and said source of alternating current, and actuated by the anode current of said tl'syratron at the end of the measuring process, each of said two timemeasuring devices comprising a constant speed motor, an electromagnetic clutch, an output shaft, means to keep said clutch deencrgiz-cd before the measuring process, and means operativeLv connected to said relay contacts to energize said clutch during and deenergize it at the end of the measuring process, and means urging said shait to return to its starting position, means maintainingsald shaft after the end of the measuring process in its most advanced position against said urging means, and means under control 01' the operator to render said last named means inoperative for resetting purposes 14. A print control instrument according to claim 1, including a housing for said photoelectric cell, adapted to be placed on said easel, said housing having an aperture in its top wall, and means to adjust the size of said aperture. whereby the sensitivity 0! said photoelectric cell to light can be controlled 15. A print control instrument according to claim 1, including a housing for said photoelectric cell, said housing having an aperture in its top wall, and electromagnetic means to change the size of said aperture, said electric circuit comprising means to shortcircuit said condenser before the start of the measuring process, a source of current in series with said condenser and said photoelectric cell, means to render said means to shortcircuit inoperable alter the start of the measuring process, whereby said condenser accumulates a charge which increases during the measuring process at a rate of speed in accordance with the intensity of the light impinging upon said photoelectric cell, means to indicate when said charge equals a voltage of a predetermined magnitude, each of said two time-measuring devices comprising a constant speed motor. 

